The present invention relates to an image processing apparatus and, more particularly, to an image processing apparatus for adjusting, at a high speed, the spatial frequency characteristic of a two-dimensional image sensed by an electronic camera apparatus when the image is to be compressed and recorded at a high speed.
An electronic camera apparatus (digital still camera) for sensing an image using an image sensing section such as a CCD adjusts the spatial frequency characteristic of an obtained two-dimensional image and compresses the image.
For example, an image obtained by a CCD using a color filter is constructed by a number of pixels having luminance values of different color components such as R, G, and B components. For this reason, pixels of the same color component are arranged in a checkerboard pattern, and actual resolution with respect to the number of pixels is low.
A general electronic camera apparatus executes processing of adjusting the spatial frequency characteristic of an input image whereby edge enhance or other image processing is performed for the input image to compensate for such deterioration in resolution.
In recent years, resolution tends to be improved by increasing the number of pixels of a CCD, and an enormous memory capacity is required to store (record) an obtained image.
Hence, in the general electronic camera apparatus, image compression processing is performed to reduce the data capacity using two-dimensional orthogonal transform without largely damaging the quality of an input image.
FIG. 20A shows the arrangement of a conventional electronic camera apparatus.
An image (RGB) photoelectrically converted by an image sensing section 11 constituted by a CCD is digitized by a signal conversion section 12. After various conversion processes including luminance/color difference signal conversion, color temperature correction, and gamma correction are executed, the image is temporarily stored in a memory 13.
An image processing apparatus 15 has a signal processing section 16 for adjusting the spatial frequency characteristic of the sensed image, a two-dimensional orthogonal transform section 18 for performing two-dimensional orthogonal transform for the image with the adjusted spatial frequency characteristic, and a quantization/compression section 18B for quantizing and compressing the transformed image data.
The signal processing section 16 is constructed by a general-purpose circuit section such as a DSP (Digital Signal Processor).
In this case, on the basis of an instruction from a control section 16B, image data 14 is sequentially read out from the memory 13, and predetermined floating-point operation is repeatedly executed using a filter coefficient 16A having a desired filter characteristic for spatial frequency characteristic adjustment to generate an intermediate image 17 having an adjusted spatial frequency characteristic.
Ito The two-dimensional orthogonal transform section 18 is constructed by a general circuit section for performing two-dimensional orthogonal transform processing such as DCT (Discrete Cosine Transform).
In this case, two-dimensional orthogonal transform processing for the intermediate image 17 is performed using a transform coefficient 18A for image data compression.
After that, the quantization/compression section 18B quantizes and compresses the image data that has undergone two-dimensional orthogonal transform to obtain compressed image data 19.
In this conventional image processing apparatus, complex floating-point operation is repeatedly executed using the signal processing section constructed by the general-purpose DSP, thereby adjusting the spatial frequency characteristic of an input image.
For this reason, the spatial frequency characteristic cannot be arbitrarily adjusted on the basis of a desired characteristic because of limitations on the time required for calculation and power consumption.
Generally, in adjusting the spatial frequency characteristic in units of matrix spaces each constructed by Mxc3x97M pixels (M is an odd number; Mxe2x89xa73) of an input image, the spatial frequency characteristic can be arbitrarily adjusted in a wide frequency band by increasing the space size M.
However, the number of times of calculation by the DSP increases in proportion to the square of M, and the processing time and power consumption also increase.
In conventional image processing, to reduce the number of times of calculation, relatively simple adjustment is performed for the spatial frequency characteristic. More specifically, the spatial frequency characteristic is adjusted in units of small matrix spaces with a size of 3xc3x973 pixels (M=3).
Hence, the degree of freedom of a characteristic in adjusting the spatial frequency characteristic is considerably limited, and no satisfactory image processing can be realized.
The present invention has been made to solve the above problem, and has as its object to provide an image processing apparatus capable of adjusting the spatial frequency characteristic of an input image and compressing the image using a desired filter characteristic.
In order to achieve the above object, according to the present invention, there is provided an image processing apparatus which, for an input two-dimensional image constructed by arraying, in a matrix, a number of pixels each representing a luminance value at a corresponding position, adjusts a spatial frequency characteristic on the basis of a desired spatial frequency adjustment filter characteristic, and compresses the image, comprising a spatial filter for obtaining a new luminance value of each pixel of an input image on the basis of a first coefficient having a first filter characteristic which forms the desired spatial frequency adjustment filter characteristic in cooperation with a second filter characteristic, thereby generating a two-dimensional intermediate image which has the adjusted spatial frequency characteristic of the input image, and an orthogonal transform section for performing two-dimensional orthogonal transform for the intermediate image on the basis of a second coefficient which has the second filter characteristic, which forms the desired spatial frequency adjustment filter characteristic in cooperation with the first filter characteristic, and is used for two-dimensional orthogonal transform for compressing the intermediate image generated by the spatial filter, thereby generating image data which has the adjusted spatial frequency characteristic of the intermediate image and has undergone two-dimensional orthogonal transform for image compression.